1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to systems and methods of providing distributed Raman amplification of optical signals.
2. Statement of the Problem
Many communication companies use optical networks for transmitting data because of its high-bandwidth capacity. Optical networks reliably transport optical signals over long distances. Optical networks include transmission fiber, transmitters, receivers, repeaters, etc. As the optical signals travel over the transmission fiber, the optical signals disperse or otherwise attenuate due to Rayleigh scattering. The attenuation may be recovered by an optical amplifier.
Optical amplifiers may be discrete amplifiers or distributed amplifiers. A distributed amplifier uses the transmission fiber carrying the optical signals as a gain medium. A discrete amplifier does not use the actual transmission fiber as the gain medium, but is a separate component that includes a span of rare-earth doped fiber, Dispersion Compensating Fiber (DCF), Highly Nonlinear Fiber (HNF), or another type of fiber as the gain medium.
In a distributed amplifier, a pump laser transmits a laser onto the transmission fiber concurrently as the optical signals travel over the fiber. The pump laser generally backward pumps the laser onto the transmission fiber. Other distributed amplifiers have a backward pumping laser and a forward pumping laser. The properties of the transmission fiber act to absorb the pumped laser and generate a gain in the optical signals due to the Raman Effect. The gain range of the distributed amplifier is flexible and depends on the wavelength of the pump laser. The pump laser amplifies wavelengths at one Raman Stokes shift away from the laser wavelength. A first order Raman Stokes shift comprises the wavelengths about 100 nanometer (nm) longer than the pump laser wavelength in glass fiber. For instance, a 1455 nm pump laser wavelength amplifies optical signals having wavelengths around 1550 nm. The gain bandwidth is about 30 nm centered about the 1550 nm wavelength.
One problem with current distributed amplifiers is that high pump powers are needed to achieve the desired gain. The pump power for distributed amplification can range from 500 milliwatts (mW) to over 1 Watt. As a comparison, a discrete Erbium-Doped Fiber Amplifier (EDFA) may have pump powers around 100 mW to achieve the same gain. Unfortunately, the high pump powers may increase the chance of connector burns, fiber fuse, and ignition of a fire. The high pump powers can also be dangerous for the operators, as the high pump powers can be harmful to the eyes of the operators and can cause severe burns.